In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. Such a light-emitting element has a structure where a layer including a substance with a light-emitting property is interposed between a pair of electrodes. By application of voltage between the pair of electrodes, light emission can be obtained from the substance with a light-emitting property.
There are various possible applications of such a self-luminous light-emitting element. For example, such a self-luminous light-emitting element is preferably used for a flat panel display element because of having advantages such as high visibility of pixels as compared to a liquid crystal display and no backlight required. Further, it also has great features and advantages that such a self-luminous light-emitting element can be manufactured to be thin and lightweight and has very high response speed with respect to an input signal.
Furthermore, since such a light-emitting element can be formed in a film form, a planar light-emitting device with a large area can be easily formed. This is a feature which is difficult to be obtained in point light sources typified by an incandescent lamp and an LED or line light sources typified by a fluorescent bulb. In addition, such a light-emitting element has attracted attention as a next-generation lighting device because it is estimated to have higher emission efficiency than filament bulbs or fluorescent bulbs.
The light-emitting elements using electroluminescence are roughly classified in accordance with whether they include an organic compound or an inorganic compound as a substance with a light-emitting property. The light-emitting element using electroluminescence which includes an organic compound as a substance with a light-emitting property emits light in the following manner. First, voltage is applied to a first electrode and a second electrode which constitute a pair of electrodes, so that electrons and holes are transported to a light-emitting layer including a substance with a light-emitting property. The electrons and holes are recombined in the light-emitting layer and energy is generated. The substance with a light-emitting property included in the light-emitting layer is excited by the energy. When the excited substance with a light-emitting property returns to a ground state, light is emitted.
Such a light-emitting element which emits light by the transport of electrons and holes to the light-emitting layer is called a current excitation type light-emitting element. Note that the excited state of an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are many problems which depend on substances. Therefore, improvement in an element structure, development of a substance, and the like have been carried out in order to solve the problems. In terms of improving efficiency, it is preferable that the light-emitting element be in a state where the numbers of electrons and holes which are transported to the light-emitting layer are balanced, i.e., a state where the balance of carriers is achieved, and a state where the transported electrons and holes are successively recombined and thus energy is generated, for example.
As an example of a method for achieving the balance of carriers in the light-emitting element, there is a method for achieving the balance of carriers transported to the light-emitting layer by the control of materials and thicknesses of a hole-transport layer provided between an anode and the light-emitting layer in the light-emitting element, and an electron-transport layer provided between the light-emitting layer and a cathode in the light-emitting element.
Further, in Non-Patent Document 1, a method for preventing holes from leaking from a light-emitting layer to a cathode side with the use of a hole-blocking layer provided between the light-emitting layer and the cathode is disclosed. The holes are trapped in the light-emitting layer, whereby the recombination of the electrons and the holes in the light-emitting layer is facilitated. As a result, the emission efficiency of a phosphorescent light-emitting substance is successfully increased.